gwd math 19-2 & 26

我这两题不是很明白， 哪位可帮我解答一下。谢了。

Q2:

For which of the following functions f is f(x) = f(1-x) for all x?

A.   f(x) = 1 - x

B.    f(x) = 1 - x²

C.   f(x) = x² - (1-x)²

D.   f(x) = x²(1-x)²

E.    f(x) = x / (1-x)

Q26:

How many different 6-letter sequences are there that consist of 1 A, 2 B’s, and 3 C’s ?

A.     6

B.     60

C.     120

D.     360

E.      720

2. The answer is D since f(1-x) = (1-x)^2(1-1+x)^2 = x^2(1-x)^2 = f(x).

26. The answer is B. 6!/2!3! = 60.